In most DC power supply applications, it is desirable to produce output voltage levels which do not vary. Undesirable variations can be produced by changes in input voltage levels, changes in loads on the supply, and temperature induced changes in component characteristics. In principle, these variations can be virtually eliminated by closed loop regulation. In addition to these types of variations, there are unwanted variations associated with the pulsating or discontinuous waveforms caused by the switching/rectifying function being performed within the supply (i.e., ripple). The conventional technique used to reduce the amplitude of an intolerable ripple is to enlarge the inductors and capacitors in each output. This approach naturally leads to bulky components.
It is possible to reduce the total volume of the magnetic components, and to reduce the number of components, by winding all the output inductors in a multi-output supply on one core. The resulting coupled inductor is not intended to provide any additional ripple reduction beyond uncoupled inductors. Indeed, such coupled inductors are constructed with turns ratios which preclude obtaining significant ripple reduction. These coupled inductors may provide some performance advantages related to cross regulation, and, in principle, provide at least some ripple reduction compared to uncoupled inductors.
Inductors coupled to deliberately obtain substantially reduced ripple have been used in some versions of the Cuk converter (see U.S. Pat. No. 4,184,197). This converter has an inductor in both the input and output circuits, so that by proper coupling, it is possible to obtain negligible ripple in either the input or output current. Extensions of coupled inductor Cuk converters include: (1) coupling of both input and output inductors to an intermediate transformer to obtain negligible ripple in both input and output inductors simultaneously and (2) coupling two output filter inductors to obtain negligible ripple in one output.
The body of background art concerned with the use of coupled inductors constructed to produce negligible ripple is constrained by
(1) use with only the Cuk converter; PA1 (2) pairwise use, either input-output or two outputs and PA1 (3) impractical implementations, which start with a fixed turns ratio and then minimize ripple by either adjusting the size of the air gap in the magnetic core, or by adding an uncoupled inductor of appropriate value in series with one of the windings. PA1 (1) the DC current could be placed nearly entirely in the shunt inductors, thus allowing the coupled windings to be constructed of light wire on a smaller core, or PA1 (2) the DC current could be placed mainly in the coupled windings, thus allowing the tailoring or "trimming" shunt inductors to be of low current construction.